Bodies of water are commonly sanitized with either a chlorine or bromine biocide. Chlorine biocides are irritating to human mucous membranes and tend to have a strong odor, while bromine biocides are not so irritating, and usually have less odor. Further, bromine biocides are known to be more effective in water than are chlorine biocides. While this suggests that bromine biocides would be favored over chlorine biocides, it is also known that bromine biocides, unlike chlorine biocides, are unstable when exposed to ultraviolet light, making bromine biocides particularly unsuitable for certain applications, such as swimming pools. In order to utilize bromine biocides, an effective method for their stabilization is needed. Introducing to the body of water a reagent which stabilizes the bromine biocide would be an appropriate way to gain the advantages of a bromine biocide. In addition to effectively stabilizing bromine even under exposure to ultraviolet light, the stabilizer must itself be nonhazardous and nonirritating to humans. Moreover, it would be of considerable advantage if the stabilization could be achieved by use of a minimum number of chemical agents, and if the biocidal activity in the water could be accomplished with as little change as possible in routine maintenance operations.